You won't see significant power gains. Maybe around 20HP/20TQ. If you drive "normally" your MPG stay about the same. However, with exhaust, you will mash on the gas pedal. I know I do!

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Jeff
Current:
'07 Jeep Wrangler
'06 BMW X5 3.0i

Former:
'12 WRX; '08 WRX; '06 BMW X5 4.8is Dinan S1; '02 BMW 530i Dinan S2